It is important to note that even in a contract marriage, consent is a compulsory requirement, although the extent to which that requirement is observed in most marriages is highly questionable. In nikkah, if either the man or the woman refuses to consent to the arrangement, the nikkah is null and void.
Also, do you mean to suggest you are against a prime tenet of Islam? That's not an advisable position to take in an increasingly ultra-religious society like Pakistan. I can't say whether I agree or disagree because the requirement of the veil is an essential component of the Islamic way of life and, therefore, my opinion on the matter would be to no end.
Reverting back to the topic, here's a concise comparison of both the arrangements:
Requirements of a nikkah:
1. Consent, in the absence of which the marriage is void ab initio;
2. Capacity to contract, although in the cases of child marriages the parents can contract on behalf of the children and when the child reaches the age where he/she develops the capacity, they can either choose to accept the marriage or revoke it in which case it stands dissolved.
3. Witnesses;
4. Written contract;
5. Permission of the Wali;
6. Money - Haq-Mehr;
Practices usually observed in prostitution
1. Consent of the sex-worker, in the absence of which it becomes statutory rape;
2. Capacity to contract. Most sex-workers engaged in the business are mature enough to understand the simple arrangement which they are entering into. Sex for money.
3. Witnesses are only a requirement where proof of the contract is required and is only true for cases where prostitution is legal and entirely oral e.g. countries like Amsterdam.
4. Agreements with sex-workers are usually oral. In countries where prostitution is illegal, the contract itself is without legal force but that does not negate the fact that an illegal 'arrangement' has nonetheless been entered into;
5. No requirement. This is the only way in which prostitution differs from nikkah. Also as you rightfully pointed out, nikkah agreements tend to be of a longer duration than contracts with prostitutes;
6. Money is given to the sex-worker in exchange for the service.
With the absence of regulatory law which governs and protects sex-workers, as is the case for marriage, the two arrangements bear resounding similarities, so much so that it may appear to shock the senses and that is completely understandable.